1. Field of the Invention
The present invention relates to a light emitting device and a method of manufacturing the light emitting device, and more particularly to, a light emitting device including a transparent electrode and a method of manufacturing the light emitting device.
2. Description of the Related Art
Recently, in order to improve performance of a white-light LED used for various applications such as a backlight source of an LCD display, solid-state lighting (SSL), and flashlight, high efficiency/high power GaN based LEDs have been actively researched.
However, in a lateral-conducting GaN-based LED having a simple structure which is widely used in the related art, since anode and cathode are designed to be formed in the same direction, a current concentration effect (current-crowding effect; CCE) occurs in the n-type electrode, and particularly, a thermal problem occurs due to insulating properties of sapphire used for a substrate. Therefore, the lateral-conducting GaN-based has a limitation in improving output power of the LED by injecting high current.
In order to solve the above-mentioned problems of the lateral-conducting GaN-based LED, a vertical-conducting structure LED (VLED) where electrodes are designed to be formed above and below the LED has been researched. In addition, a vertical-conducting structure metal-substrate GaN LED (VM-LED) where a sapphire substrate is lifted off by using laser (laser lift-off; LLO) and a metal is used as a substrate for efficiently releasing heat has been researched.
In comparison to the lateral-conducting structure GaN-based LED, the performance of the VM-LED is greatly improved. Namely, the output power PO of the VM-LED is increased, and the operating voltage (forward voltage; VF) thereof is decreased. However, in the manufacturing of the high-power LED, there are still the problems of current concentration (current crowding) in the portion below the n-type metal contact pad into current is injected, non-uniform current dispersion, and non-uniform light emission.
In order to solve the problems, in many research groups, an N-face n-GaN layer where a convex-concave pattern is formed and metal are formed to be in good ohmic contact to implement uniform current injection and uniform current dispersion in the n-type electrode of the VLED. In addition, in many research groups, the n-type electrode is efficiently designed to implement uniform current dispersion and uniform current injection.
In order to implement uniform current dispersion and injection, the contact area of the n-electrode is allowed to be increased (n-electrode pattern design), or a current blocking layer (CBL) is used. In addition, a transparent conduction layer (TCL) and a transparent conduction electrode (TCE) may also be inserted between the n-electrode and the GaN layer. However, the above-mentioned techniques in the related art have the problems, as follows.
First, if the contact area of the n-type electrode is increased, uniform current dispersion and injection can be advantageously implemented. However, since light emitted from an activation layer (multi quantum well; MQW) is blocked or absorbed, light extraction efficiency is deteriorated.
In the case where a current blocking layer is inserted, since the current blocking layer is formed by inserting an insulating material having almost the same size as the n-type electrode into a symmetric position of the p-GaN layer, current concentrated below the n-type electrode can be advantageously dispersed. However, since current does not flow through the current blocking layer, the current can be injected into the portion excluding the current blocking layer. Namely, since the current is not injected through the entire area of the p-GaN layer, there is a problem in terms of efficiency. In addition, since the current blocking layer needs to be formed at an accurate position with an accurate size, there is difficulty in the manufacturing process.
Finally, in the technique of inserting a transparent conduction layer, there is an advantage in that current can be effectively dispersed and injected by using an n-type electrode having a small area. However, when the light is emitted out, since the light is absorbed by the transparent conduction layer, for example, ITO (indium tin oxide), IZO (indium zinc oxide), there is a problem in that the light extraction efficiency is decreased.
In the case of the UV-LED for which demands have been greatly increased recently, the problem of decrease in light extraction efficiency becomes more serious.
FIG. 1 is a graph illustrating transmittance in the case where an ITO transparent electrode is formed on a p-GaN semiconductor layer in the related art. As illustrated in FIG. 1, the transmittance of the ITO transparent electrode is 80% or more in a wavelength range of 350 nm or more, but the transmittance is greatly decreased in a short wavelength range, that is, a UV wavelength range. Particularly, in a short wavelength range of 280 nm or less, the transmittance is decreased down to 20% or less. In order to solve the problem, in another technique in the related art, the transparent electrode is not formed on the semiconductor layer such as a p-AlGaN layer, but a metal electrode pad is directly formed thereon. However, since a different in work function between the metal and the semiconductor layer is too large, ohmic contact is not formed therebetween, and current is concentrated on the metal electrode pad so as not to spread over the entire activation layer. In other words, the above-described problems still occur.
Although various researches have been made in order to solve the above-described problems, a transparent electrode capable of solving the problem of current concentration and implementing high transmittance and high conductivity with respect to light in a UV wavelength range has not been developed. This is because conductivity and transmittance of a material has a trade-off relationship. Since a material having as high transmittance as it can be used in a UV wavelength range has a large band gap (larger than the band gap (3.4 eV) of ITO), the material has too low conductivity to be used as an electrode and is not in ohmic contact with a semiconductor material. Therefore, it is impossible to use the material as an electrode.